The invention relates to a tensioning device of a belt and chain drive with a tensioning lever that is mounted so that it can rotate on a base housing via a drag bearing constructed as a plain bearing and that is provided with a rotating tensioning roller at a radial distance from the rotational axis of the drag bearing, with a pre-tensioned torsion spring that is constructed as a helical spring and that is arranged coaxial to the drag bearing and that is connected on its spring ends on the housing side to the base housing and on the lever side to the tensioning lever, and with a brake shoe that is arranged on one spring end and that can be pressed by this spring end against a component that can rotate about the rotational axis of the drag bearing relative to the spring end.
Tensioning devices of the structure designated above are used in different constructions preferably in auxiliary assembly drives of combustion engines. Such a tensioning device is used to tension a belt and chain, such as a V-type belt or a multi-V belt that is guided over a driving wheel and usually several driven wheels, that is, to compensate for changes in length and tension caused by operation in the belt or chain and thus to prevent slippage of the belt and chain on one of the wheels.
For this purpose, the tensioning device is arranged within the belt or chain drive in the region of a non-tensioned section of the belt or chain such that the tensioning roller mounted on the tensioning lever so that it can rotate via a roller bearing is partially wrapped around by the belt or chain and is pressed against the belt or chain by the effect of a twisting moment of the pretensioned torsion spring acting on the tensioning lever. For a reduction of the tension of the belt or chain associated with lengthening of the affected belt section, the loop of the belt or chain guided around the tensioning roller is increased under the spring extension of the tensioning lever. For an increase in the tension of the belt or chain associated with shortening of the belt section, the loop of the belt or chain guided around the tensioning roller is decreased under spring retraction of the tensioning lever. Both pivoting movements of the tensioning lever are damped by the friction moment of a brake shoe, wherein, in a desired way, the damping is dependent on the direction of the load, that is, proportional to the twisting moment of the torsion spring, and thus the spring retraction of the tensioning lever is more strongly damped than the spring extension.
Such tensioning devices are known both in an embodiment mounted on the inside with a support of a bearing journal connected rigidly to the tensioning lever in the boss forming a component of the base housing and also in an embodiment mounted on the outside with a support of a boss connected rigidly to the tensioning lever on the bearing journal forming a component of the base housing. The bearing of the tensioning lever in or on the base housing is formed by at least one plain bearing bushing that is arranged between the bearing journal and the boss and that is usually made from a durable and simultaneously low-friction plastic material.
The base housing can be provided with outer mounting flanges or with a central borehole for mounting the tensioning device on a motor housing, such as, e.g., the crankcase or the timing case of a reciprocating-piston combustion engine. With respect to the arrangement of the tensioning roller, for such a tensioning device there is also a difference between a so-called offset or Z configuration in which the tensioning roller is arranged axially on the outside of the tensioning lever facing away from the base housing and a so-called in-line or U configuration in which the tensioning roller is arranged in the radial direction at the side of the base housing axially on the inside of the tensioning lever facing the base housing.
Due to tight installation relationships, general cost pressures for production, as well as high performance requirements, the tensioning device must satisfy many requirements. Thus, the tensioning device should have small dimensions, that is, should take up less structural space. For achieving low production costs, the tensioning device should be made from the fewest number of components possible that have a geometrically simple construction and thus can be produced and also assembled easily and economically. In addition, the tensioning device should feature high friction damping that can be adjusted to the corresponding application and good heat dissipation of the friction heat in this respect. Finally, by achieving uniform and therefore essentially tilting moment-free loading of the drag bearing, lower wear of the plain bearing bushing and a precise maintenance of the angle of the tensioning lever and also the tensioning roller relative to the base housing or the belt and chain should be given. In order to satisfy these requirements, different tensioning devices of the constructions noted above have already been proposed.
From EP 0 780 597 B1, a tensioning device according to the class is known in which a brake shoe provided with a relatively small, outer cylindrical friction surface is arranged on the housing-side spring end of a helical spring that can be loaded in the closing sense. The spring end has an inward angled spring leg and comprises, in the region of the corner of the angle, a housing-fixed journal by which the twisting moment of the helical spring is introduced into the base housing. The spring leg contacts the inside of the brake shoe and presses this brake shoe with its outer cylindrical friction surface in the radial direction against a cylindrical inner wall of the tensioning lever and also, on the peripheral side, against a stop surface of a housing-fixed stop that contacts the brake shoe in a first embodiment or against a housing-fixed journal, wherein, in a second variant, the brake shoe is mounted so that it can rotate about this journal.
The radial contact force of the brake shoe on the tensioning lever is provided for the purpose of, in addition to the generation of a friction moment, damping the pivoting movement of the tensioning lever in connection with the axial distance of the brake shoe to a middle radial bearing plane of the plain bearing bushing of the drag bearing, also for the compensation of a tilting moment caused by the resulting belt and chain force in connection with the axial distance of the tensioning roller. Due to the lever relationships at the spring end and the division of the spring force transmitted by the spring leg to the brake shoe, the friction moment generated in this way, however, is relatively small. In addition, the construction of the affected damping device is relatively complicated and can only be produced and assembled accordingly high expense. In addition, the introduction of the heat generated by the friction work of the brake shoe is disadvantageously limited locally and in the tensioning lever. The tensioning lever, however, is already thermally loaded by the rolling bearing of the tensioning roller and the heat of the tensioning lever is dissipated from there to the surrounding air with relatively poor heat transfer.
For avoiding at least some of the disadvantages named above, in the unpublished DE 10 2006 023 565.7 of the applicant, alternative embodiments of a tensioning device according to the class have been proposed. In a first preferred embodiment of the tensioning device according to FIG. 5 in this document, a brake shoe provided essentially as a circular ring segment with a relatively large outer cylindrical friction surface is arranged on the housing-side spring end of a helical spring that can be loaded in the closing sense between the outer winding of the spring end and a cylindrical inner wall of the tensioning lever. The spring end has a spring leg that is bent outward and that contacts the free end of the brake shoe on the peripheral side. The opposite end of the brake shoe on the peripheral side contacts a stop surface of a housing-fixed catch. The twisting moment of the helical spring is thus introduced as a tangential force via the spring leg, the brake shoe, and the catch into the base housing, wherein a relatively large radial contact force is produced from the vector addition of the two tangential forces acting on the brake shoe, wherein, with this contact force, the brake shoe is pressed with its friction surface against the cylindrical inner wall of the tensioning lever.
In a second embodiment of the tensioning device according to FIG. 6 in DE 10 2006 023 565.7, a brake shoe similarly provided as a circular ring segment with a relatively large outer cylindrical friction surface is arranged on the housing-side spring end of a leg-less helical spring that can be loaded in the opening sense facing away from the outer winding of the spring end on a cylindrical inner wall of the tensioning lever. The stub spring end of the helical spring contacts, on the peripheral side, the free end of the brake shoe. The opposite end of the brake shoe on the peripheral side contacts a stop surface of a housing-fixed catch. In this case, the twisting moment of the helical spring is introduced as a tangential force via the stub spring end, the brake shoe, and the catch into the base housing, wherein a relatively large radial contact force is also produced from the vector addition of the two tangential forces acting on the brake shoe, wherein, with this contact force, the brake shoe is pressed with its friction surface against the cylindrical inner wall of the tensioning lever.
The damping devices of the two configurations of the known tensioning device noted above have a construction that can be produced more easily and more economically relative to the previously known tensioning device described above, a larger surface-area introduction of the friction heat of the brake shoe, and a significantly increased friction damping of a pivoting movement of the tensioning lever due to a significantly increased contact force. However, the disadvantage of the introduction of the friction heat of the damping device into the tensioning lever remains.